1. Field of the Invention
The invention relates to novel triazole derivatives with improved receptor activity and bioavailability properties that act as ghrelin analogous ligands of growth hormone secretagogue receptors. These compounds are useful in modulating growth hormone plasma levels in mammals as well as in the treatment and/or regulation of various physiological and pathophysiological conditions, such as growth retardation, obesity, food intake, energy balance, and other metabolic disorders, tumor cell proliferation, wound/burn/bone healing, inflammation, and addiction processes like food reward, alcohol-related disorders and drug abuse.
2. Description of the Related Art
Ghrelin, a 28 amino acid peptide with a unique octanoyl modification on Ser-3 (Kojima M et al., Nature 1999, 402: 656-660), was identified as an endogenous ligand for the growth hormone secretagogue receptor type 1a (GHS-R 1a), a G-protein coupled receptor (Howard A D et al., Science 1996, 273: 974-977). Ghrelin is essentially produced in the upper intestinal tract/stomach but lower amounts were also detected in bowel, pancreas, kidney, the immune system, placenta, testes, pituitary, lung and in the hypothalamus (van der Lely A J et al., Endocrine Rev. 2004, 25: 426-457; Cowley M et al., Neuron 2003, 37: 649-661).
In humans, ghrelin stimulates growth hormone (GH) via a pathway independent from GHRH receptor and in synergy with GHRH on GH secretion (Arvat E et al., J. Clin. Endocrinol. Metab. 2001, 86: 1169-1174). Besides, it also stimulates ACTH, prolactin, cortisol, aldosterone and epinephrine secretion (Arvat E et al., J. Clin. Endocrinol. Metab. 2001, 86: 1169-1174; Nagaya N et al., Am. J. Physiol. Regul. Integr. Comp. Physiol. 2001, 280: R1483-1487; Takaya K et al., J. Clin. Endocrinol. Metab. 2000, 85: 4908-4911).
Ghrelin is thought to participate in metabolism regulation and energy expenditure, so ghrelin expression and secretion into the general circulation from the stomach is expected to be influenced by metabolic hormones. In obese humans, plasma ghrelin levels are reduced, suggesting that the elevated insulin or leptin levels of obese subjects lower ghrelin secretion (Tschop M et al, Diabetes 2001, 50: 707-709).
The release of growth hormone in humans and animals is believed to treat physiological or pathophysiological conditions characterized by a deficiency in growth hormone secretion as well as to treat those conditions which are improved by the anabolic effects of growth hormone.
Initially, clinical applications with GH were limited to treatment of GH-deficient children, but the commercialization of recombinant human growth hormone (rhGH) allowed many studies which showed other potential clinical uses of GH (Strobl J S et al., Pharmacol. Rev. 1994, 46: 1-34; Torosian M H, J. Pediatr. Endocrinol. 1993, 6: 93-97). rhGH has shown promise in the treatment of patients with burns, wounds, bone fractures and more recently in reversing the catabolic effects of glucocorticoids, chemotherapy and AIDS as well as in modifying body composition (Rudman D et al., N. Engl. J. Med. 1990, 323: 1-6; Papadakis M A et al., Ann. Intern. Med. 1996, 124: 708-716; Welle S et al., J. Clin. Endocrinol. Metab. 1996, 81: 3239-3243).
GH, synthesized and stored in the pituitary gland, is released under the control of two known hypothalamic hormones: growth hormone releasing hormone (GHRH) and the inhibitory hormone somatostatin (SRIF). In most cases, GH deficiency is related to a hypothalamic defect and not to a pituitary deficiency in GH. Therefore, as an alternative treatment to rhGH, GH-deficient patients could also be treated with any compound that releases endogenous GH from the pituitary gland. This can either be performed with GHRH which stimulates GH release but also with synthetic growth hormone secretagogues (GHS).
Many synthetic, peptidyl and non-peptidyl GHS, such as GHRPs 1, 2 and 6, Hexarelin, MK-0677, EP-01572, were shown to specifically bind to the then orphan receptor “GHS receptor”—several of them long before ghrelin and ghrelin/GHS receptor were discovered (see “Camanni F et al., Front Neuroendocrinol. 1998, 19: 47-72”; “Casanueva F F et al., Trends Endocrinol. Metab. 1999, 10: 30-38”; “van der Lely A J et al., Endocrine Rev. 2004, 25: 426-457” for further references). GHS also show potent GH releasing action and have the same biological activities as mentioned above for ghrelin.
GHS were also disclosed in the following patents or patent applications (not exhaustive list): U.S. Pat. No. 6,071,926, U.S. Pat. No. 6,329,342, U.S. Pat. No. 6,194,578, US 2001/0041673, U.S. Pat. No. 6,251,902, US 2001/0020012, US 2002/0013320, US 2002/0002137, WO 95/14666, WO 96/15148, WO 01/96300.
While the ghrelin/GHS induced GH secretion is mediated by the activation of the ghrelin/GHS receptor type 1a (GHS-R 1a), there is evidence so far that at least some of the other effects of ghrelin and GHS are also mediated by different receptors of the GHS receptor family or even different binding sites on a given GHS receptor.
GHS receptors are concentrated in the hypothalamus-pituitary area but appear also to be distributed in other central and peripheral tissues (Hattori N et al., J. Clin. Endocrinol. Metab. 2001, 86: 4284-4291; Gnanapavan S et al., J. Clin. Endocrinol. Metab. 2002, 87: 2988-2991; Muccioli G et al., J. Endocrinol. 2000, 157: 99-106; Muccioli G et al., Ann. Endocrinol. 2000, 61: 27-31; Muccioli G et al., Eur. J. Pharmacol. 2002, 440: 235-254; Papotti M et al., J. Clin. Endocrinol. Metab. 2000, 85: 3803-3807; Cassoni P et al., J. Clin. Endocrinol. Metab. 2001, 86: 1738-1745; Guan X M et al., Brain Res. Mol. Brain. Res. 1997, 48: 23-29; Bluet-Pajot M T et al., Endocrine 2001, 14: 1-8; Korbonits Metal., J. Clin. Endocrinol. Metab. 1998, 83: 3624-3630).
Two GHS type 1 receptors have been identified, GHS-R 1a and GHS-R 1b, that in human are presumably expressed by a single gene and alternatively spliced (van der Lely A J et al., Endocrine Rev. 2004, 25: 426-457; Howard A D et al., Science 1996, 273: 974-977; Smith R G et al., Endocr. Rev. 1997, 18: 621-645; Smith R G et al., Endocrine 2001, 14: 9-14; McKee K K et al., Mol. Endocrinol. 1997, 11: 415-423; Petersenn S, Minerva Endocrinol. 2002; 27: 243-256). Among mammalian species a high degree of sequence identity has been reported for GHS-R 1a (Petersenn S, Minerva Endocrinol. 2002; 27: 243-256: between 91.8% and 95.6%).
Motilin receptor, was discovered as a member of the GHS receptor family, having 52% identity (Smith R G et al., Endocrine 2001, 14: 9-14; McKee K K et al., Genomics 1997, 46: 426-434). Gastrointestinal motilin receptor 1a and GHS-R 1a show a high similarity (Smith R G et al., Endocrine 2001, 14: 9-14; Feighner S D et al., Science 1999, 284: 2184-2188).
Other GHS receptor family members appear to be neurotensin receptor, TRH receptor, GPR38 (FM1), GPR39 (FM2) and FM3 (Smith R G et al., Endocr. Rev. 1997, 18: 621-645; Smith R G et al., Horm. Res. 1999, 51 (Suppl. 3): 1-8; Tan C P et al., Genomics 1998, 52: 223-229; Howard A D et al., Science 1996, 273: 974-977). Further GHS receptor subtypes appear to exist in a wide variety of central and peripheral tissues (van der Lely A J et al., Endocrine Rev. 2004, 25: 426-457). For instance, a cardiac GHS-R has been reported (Bodart V et al., Circ. Res. 1999, 85: 796-802) with a predicted sequence similar to that of CD36, a multifunctional receptor known as glycoprotein IV (Bodart V et al., Circ. Res. 2002, 90: 844-849). Cassoni et al. (J. Clin. Endocrinol. Metab. 2001, 86: 1738-1745) report the existence of GHS-R subtypes in neoplastic mammary cells that are activated by ligands binding to specific binding sites different from the classical GHS-R type 1. Furthermore, data gathered by these authors support the hypothesis that even different binding site subtypes do exist for GHS-R in peripheral organs, which are possibly due to their endocrine or non-endocrine, but also on their normal or neoplastic nature.
The ubiquity of GHS binding sites explains that independently from their strong growth hormone secretagogue properties, ghrelin as well as synthetic GHS are implicated in several important physiological and pathophysiological conditions.
Accordingly, potential clinical applications include among others                a) Short-, Medium- and Long-Term Regulation of Energy Balance and/or Food Intake (Tschoep M et al., Nature 2000, 407: 908-913; Asakawa A et al., Gut 2003, 52: 947-952; US 2001/0020012; Kojima M et al., Curr. Opin. Pharmacol. 2002, 2: 665-668; Horvath T L et al., Curr. Pharm. Des. 2003, 9: 1383-1395; Wren A M et al., J. Clin. Endocrinol. Metab. 2001, 86: 5992-5995)                    Expression of GHS-R1a has been shown on neurons of hypothalamus paraventricular nucleus. These neurons send efferents onto key hypothalamic circuits for the control of food intake, like the arcuate nucleus which produces the mediator NPY. It is thought that the stimulation of food intake by ghrelin and/or GHS is mediated by an increase of NPY in the arcuate nucleus (Willesen M G et al., Neuroendocrin. 1999, 70: 306-316). Single administration (icv or ip) of anti-ghrelin IgG suppressed acute feeding in lean rats (Bagnasco M et al., Regul. Pept. 2003, 111: 161-167). Chronic twice-daily icv administration of anti-ghrelin IgG reduced body weight over a five-day period (Murakami N et al., J. Endocrinol. 2002, 174: 283-288).            A recent study using a peptidic GHS-R 1a antagonist, [D-Lys-3]-GHRP-6, showed a reduction of food intake and body weight gain in diet induced obese mice (Asakawa A et al., Gut, 2003, 52: 947-952). The fact that peptidyl compounds, initially characterized as growth hormone secretagogues, are able to stimulate selectively food intake in rats without inducing growth hormone secretion, suggests the existence of a GHS-R subtype different from GHS-R 1a in the hypothalamus (Torsello A et al., Neuroendocrin. 2000, 72: 327-332; Torsello A et al., Eur. J. Pharmacol. 1998, 360: 123-129).                        b) Treatment of Adipogenesis, Adiposity and/or Obesity and Reduction of Body Weight (Tschop M et al., Nature 2000, 407: 908-913; Asakawa A et al., Gut 2003, 52: 947-952)                    Chronic administration of ghrelin and/or GHS in freely feeding mice and rats results in increased body weight and decreased fat utilization (Tschop M et al., Nature 2000, 407: 908-913). Furthermore, it has been reported that ghrelin and des-octanoyl ghrelin promote adipogenesis in vivo (Thompson N M et al., Endocrinol. 2004, 145: 234-242) and inhibit isoproterenol-induced lipolysis in rat adipocytes via a non-type GHS-R 1a (Muccioli G et al., Eur. J. Pharmacol. 2004, 498: 27-35). On the other hand, there is also a report describing that the expression of the GHS-R1a in rat adipocytes increases with age and during adipogenesis (Choi K et al., Endocrinol. 2003, 144, 754-759).                        c) Treatment of Tumor Cell Proliferation                    As in the case for other members of the hypothalamus-pituitary axis which regulates the secretion of growth hormone, evidence is emerging to indicate that ghrelin and GHS-receptors may play an important autocrine/paracrine role in some cancers (Jeffery P L et al., Cytokine Growth Factor Rev. 2003, 14: 113-122). Specific binding sites for ghrelin, peptidyl- and non-peptidyl GHS are present in tumoral tissues, like prostate cancer cell line PC3 (Jeffery P L et al., J. Endocrinology 2002, 172: R7-R11), thyroid tissue (Cassoni P et al., J. Endocrinol. 2000, 165: 139-146), lung carcinoma cells CALU-1 (Ghé C et al., Endocrinol. 2002, 143: 484-491) and breast carcinomas (Cassoni P et al., J. Clin. Endocrinol. Metab. 2001, 86: 1738-1745).            In the case of breast, the specific binding sites for GHS were found in tumoral tissue while the normal mammary parenchyma did not reveal such receptors. Synthetic GHS have been reported to inhibit the proliferation of lung carcinoma cells CALU-1 (Ghé C et al., Endocrinol. 2002, 143: 484-491) and that of breast carcinoma cell lines (Cassoni P et al., J. Clin. Endocrinol. Metab. 2001, 86: 1738-1745).            Both ghrelin and non-acylated ghrelin bind to tumoral tissues. Because non-acylated ghrelin is unable to bind the GHS-R 1a, it is likely that the binding site of GHS to tumoral tissues is different from the GHS-R 1a. From these data, one can anticipate that the binding site in tumoral tissues recognizes ligands of the GHS-R 1a and in addition other not yet characterized chemical structures. Synthetic ligands of GHS-R1a may have therefore the potential to inhibit the proliferation of tumor cells expressing subtypes of GHS receptors.                        d) Treatment of Inflammation/Anti-Inflammatory Effects                    The anti-inflammatory effect of the ghrelin agonist growth hormone-releasing peptide-2 (GHRP-2) in chronic arthritis with clinical manifestations of hypermetabolism and cachexia was demonstrated (Granado M et al., Am. J. Physiol. Endocrinol. Metab. 2005, 288: E486-492). These data suggest that the anti-inflammatory action of GHRP-2 is mediated by activation of ghrelin receptors expressed by immune competent cells.                        e) Treatment of Cachexia                    The anti-cachetic effect of administered recombinant growth hormone in an animal model of chachexia (Roubenoff R et al., Arthritis Rheum. 1997, 40(3): 534-539) could be demonstrated (Ibanez de Caceres I et al., J. Endocrin. 2000, 165(3): 537-544). The findings are also in line with data of patients with rheumatoid arthritis (Roubenoff R et al., J Clin Invest. 1994, 93(6): 2379-2386).                        f) Treatment of Gastrectomy (Ghrelin Replacement Therapy)                    The gastric hormone ghrelin was given to mice subjected to gastrectomy or sham operation (Dornonville de la Cour C et al., Gut 2005, 54(7): 907-913). The results presented show that ghrelin replacement therapy at least partially reverse gastrectomy induced reduction in body weight and body fat.                        g) Treatment of (Gastric) Postoperative Ileus                    The effect of ghrelin on the motor function of the gastrointestinal tract in rat was evaluated. It could be shown that ghrelin reverses the delayed gastric evacuation and is a strong prokinetic agent useful for the treatment/reversion of postoperative gastric ileus (Trudel L et al., Am J Physiol Gastrointest Liver Physiol 2002, 282(6): G948-G952).                        h) Treatment of Diabetes (Diabetes Type I and Type II)                    The effect of ablation of ghrelin in leptin-deficient mice was studied (Sun et al., Cell Metabolism 2006, 3: 379-386). The results show that deletion of ghrelin augments insulin secretion in response to glucose challenge indicating that inhibition of ghrelin or counteracting its activity may be a possible way for the treatment of diabetes including its subtypes I and II (see also WO 03/051389).                        i) Treatment of Addiction Processes Like Food Reward, Alcohol Disorders, and Drug Abuse                    The mesolimbic dopamine projections, originating from neuronal cell populations in the ventral tegmental area (VTA) and terminating in the ventral striatum and the prefrontral cortex, are linked to anticipatory, appetitive or approach phases of motivated behaviour and are important for anticipatory food reward and food seeking behaviours (Bassareo and Chiara 1999, Neuroscience 89, 637-641; Richardson and Gratton 1998, J Neurosci 18, 9130-9138). Activation of these dopamine projections is also elicited by ingestion of rewarding foods as well as by other rewards, both natural (e.g. sex) and artificial, like alcohol and drug abuse (Berridge and Robinson 1998, Brain Res Brain Res Rev 28, 637-641). There is accumulating evidence that the mesolimbic system is a target for ghrelin. In addition to the hypothalamus, the ghrelin receptor has also been identified in the ventral tegmental area (VTA) and laterodorsal tegmental area (LDTg). More recent findings indicate that the effects of ghrelin on food intake are partly mediated by the mesolimbic dopamine systems involved in reward-seeking behaviour (Jerlhag E. et al. 2006 Addiction Biology 11:45-54; Jerlhag E, et al. Addict Biol. 2007 12:6-16; Egecioglu E. et al. 2010, Addiction Biology 15, 304-311). In addition, most recent data confirm the suitability of ghrelin antagonists for the treatment of alcohol disorders (WO2009/020419 and Jerlhag et al. 2009. PNAS 106, 11318-11323) and drug abuse (Jerlhag E. et al. 2010, Psychopharmacology 211, 415-422).                        
Further fields of application comprise acceleration of recovery of patients having undergone major surgery (e.g. U.S. Pat. No. 6,194,578); accelerating the recovery of burn patients (e.g. U.S. Pat. No. 6,194,578); attenuating protein catabolic response after a major operation (e.g. U.S. Pat. No. 6,194,578); reducing cachexia and protein loss due to acute or chronic illness (e.g. U.S. Pat. No. 6,194,578); treating central nervous system disorders of patients undergoing a medical procedure in combination with antidepressants (e.g. US 2002/0002137 A1); acceleration of bone fracture repair and cartilage growth (e.g. U.S. Pat. No. 6,194,578); treatment or prevention of osteoporosis; stimulation of the immune system; accelerating wound healing (e.g. U.S. Pat. No. 6,194,578); treatment of growth retardation associated with the Prader-Willi syndrome, Turner's syndrome and obesity; treatment of intrauterine growth retardation, skeletal dysplasia, hypercortisolism and Cushing's syndrome; treatment of osteochondrodysplasias, Noonan's syndrome, schizophrenia, depressions and Alzheimer's disease; treatment of pulmonary dysfunction and ventilator dependency; treatment of hyperinsulinemia including nesidioblastosis; adjuvant treatment for ovulation induction; prevention of the age-related decline of thymic function; improvement in muscle strength and mobility (e.g. U.S. Pat. No. 6,194,578); maintenance of skin thickness (e.g. U.S. Pat. No. 6,194,578); improvement of sleep quality (e.g. U.S. Pat. No. 6,071,926); prevention of congestive heart failure alone (e.g. U.S. Pat. No. 6,329,342; U.S. Pat. No. 6,194,578) and in combination with corticotropin releasing factor antagonists (e.g. US 2001/0041673); metabolic homeostasis or renal homeostasis (e.g. in the frail elderly)(e.g. U.S. Pat. No. 6,194,578); improving glycemic control (e.g. U.S. Pat. No. 6,251,902); treatment of systemic lupus erythematosus and inflammatory bowel disease (e.g. US 2002/0013320); treating or preventing frailty associated with aging or obesity (e.g. U.S. Pat. No. 6,194,578); as well as stimulation of osteoblasts.
Animals were not forgotten in potential applications such as stimulation of food intake (Wren A M et al., Diabetes 2001, 50: 2540-2547), stimulation of the immune system in companion animals and treatment of disorder of aging, growth promotion in livestock and stimulation of wool growth in sheep.
Compounds containing triazole moieties have been widely recognized in the medicinal chemistry due to their various biological activities. The following patent families are all directed to heterocyclic compounds that are said to show certain biological action for use in different medicinal indications. Triazole moieties are implicitly or explicitly contained.
Triazole derivatives as Ghrelin Analogue Ligands of Growth Hormone Secretagogue Receptors having good receptor affinity are disclosed in WO07/020013.
WO 2004/111015 discloses modulators of the glucocorticoid receptor. WO 2004/052280 describes anti-angiogenic compounds as inhibitors of tyrosine kinase activity of VEGF receptors and their use in cancer. WO 2004/096795 also discloses tyrosine kinase inhibitors, preferably C-FMS inhibitors. WO 03/011831 and WO 03/011210 both describe heteroarylheteroalkylamine derivatives as inhibitors of nitric oxide synthase. WO 02/00651 is directed to Factor XA inhibitors for use in thromboembolic disorders. WO 01/94318 and WO 01/94317 both describe chemical libraries of substituted azole derivatives and methods of their synthesis for use in drug discovery high-throughput screening. However, they fail to provide any biological activity or any medicinal use nor do they name specific compounds. WO 00/76971 and WO 00/76970 both claim serine protease inhibitors useful as antithrombotic agents. WO 01/36395 discloses triazole derivatives as farnesyl transferase inhibitors. WO 96/33176 and U.S. Pat. No. 5,703,092 are directed to hydroxamic acid compounds as metalloprotease and TNF inhibitors. WO 93/09095 describes 2-heterocyclicethylamine derivatives and their use in neurological and neurodegenerative disorders. WO 2004/103270 claims compounds for the treatment of thrombosis, in particular Factor XIa inhibitors. WO 98/38177, U.S. Pat. No. 6,506,782, U.S. Pat. No. 6,849,650 and US 2003/0130188 all describe heterocyclic compounds as inhibitors of beta-amyloid peptide release or its synthesis for use in Alzheimer's disease.
Heterocyclic compounds that may be useful as GHS have also been described in the literature.
WO 00/54729, for instance, discloses heterocyclic aromatic compounds as GH secretagogues which are said to stimulate endogenous production and/or release of GH and can also contain triazole moieties. In addition, a method for increasing levels of endogenous GH or increasing the endogenous production or release of GH administering such GHS is described. Furthermore, a method is provided for preventing or treating osteoporosis (improving bone density and/or strength), or treating obesity, or increasing muscle mass and/or muscle strength and function in elderly humans, or reversal or prevention of frailty in elderly humans administering such GHS.
However, although claiming in vivo GH release, WO 00/54729 fails to actually prove such effect. Neither in vitro nor in vivo data are contained that demonstrate any stimulation of or increase in endogenous production and/or release of GH.
Besides, WO 00/54729 fails to describe and show action of those claimed compounds on any biological target, i.e. claimed compounds are not shown/described to be ligands of one or more specific receptors, for instance of a receptor family, that bind to them and modulate their activity.
Furthermore, WO 00/54729 fails to describe and demonstrate inhibitory and/or antagonistic activity of claimed compounds. As a matter of fact, such compounds are not shown to decrease levels of endogenous GH and/or inhibit or decrease endogenous production and/or release of GH. Nor is an inhibitory action on any receptor mentioned nor made obvious.
U.S. Pat. No. 6,525,203, U.S. Pat. No. 6,518,292 U.S. Pat. No. 6,660,760 are members of the same patent family as WO 00/54729 that, however, do not comprise triazole moieties as claimed subject matter any more. With regard to biological activity, the above stated facts as for WO 00/54729 apply.
WO 2004/021984 describes heterocyclic aromatic compounds GH secretagogues which are said to be useful in stimulating endogenous production or release of GH. However, claimed compounds consists of bi- to tetracylic aromatic rings and do not contain triazoles.
Analogous to WO 00/54729 in vivo GH release is claimed, but neither in vitro nor in vivo data are contained that demonstrate any stimulation of or increase in endogenous production and/or release of GH. With regard to biological activity, the same stated facts as for WO 00/54729 apply.
WO 97/23508 claims compounds of peptide mimetic nature as GHS and are said to act directly on pituitary cells in vitro to release GH therefrom and show improved properties, such as improved resistance to proteolytic degradation and improved bioavailability. In addition, claimed compounds could also be administered in vivo to increase GH release. The compounds are peptide derivatives and do not explicitly contain triazole moieties.
However, once again and in analogy to above WO 00/54729 and WO 2004/021984, WO 97/23508 fails to exhibit any in vitro or in vivo data that demonstrate the claimed effects such as direct action on pituitary cells, GH release therefrom and improved properties. Furthermore, with regard to biological targets and inhibitory/antagonistic activity, the above stated facts as for WO 00/54729 apply.
U.S. Pat. No. 6,127,391, U.S. Pat. No. 5,977,178 and U.S. Pat. No. 6,555,570 are members of the same patent family as WO 97/23508. The facts as stated for WO 97/23508 do apply.
The compounds described in this invention were designed to exhibit improved activity against ghrelin receptors of at least factor three compared to a representative compound, i.e. compound 50 disclosed in WO07/020,013.
Furthermore, the compounds described in this invention are expected to possess improved oral bioavailability. Chemical synthesis was directed to modify residues R2, R5, and R7 in order to provide compounds with improved ADME properties.
In comparison to the compounds disclosed in WO07/020,013, the compounds described herein exhibit improved properties in at least one of the following parameters, which are widely accepted to be essential for reasonable oral bioavailability (Caldwell G W. 2000, Curr Opin Drug Discov Devel. 3(1), 30-41; Thomas V H. et al. 2006, Expert Opin Drug Metab Toxicol 2(4), 591-608):
CaCo-2 permeability: PAPP (a→b)>1×106 cm/s
                No evidence of efflux as indicated by b→a/a→b ratio<2Metabolic stability: In in vitro microsomal stability studies, a rat CL of <50 ml/min/kg (alternatively, >30% remaining at 60 min)        